1. Field of Invention
The present invention relates generally to device packaging and, more particularly, to lead frame packaging.
2. Description of the Background
Conventional semiconductor chip packaging techniques are not ideal for RF or microwave circuits. For example, with conventional semiconductor chip packaging techniques, an injection molded plastic package is molded onto the surface of the chip. A majority of microwave circuit chips, however, are fabricated from a gallium arsenide (GaAs) substrate, which is generally thin and fragile. Therefore, when the plastic is molded onto the surface of a GaAs microwave chip, it may damage the chip mechanically. In addition, microwave chips often include air bridges. Because plastic is a dielectric, molding plastic onto the surface of the chip may also have the effect of detuning the circuit. Another drawback of conventional semiconductor processing techniques is that the leads are typically not designed for controlled impedances. Impedance matching of the leads, however, is critical for high performance RF and microwave circuits. A further drawback of conventional semiconductor packaging techniques is that the plastic used for the packaging is very lossy at high frequencies, which may cause significant performance degradation in RF and microwave circuits.
Another problem with conventional semiconductor packaging techniques is wire bonding. Wire bonding is typically used to provide electrical connections between bonding pads of the chip and the leads of the package. Current wire bonding technology, however, prevents wire bonds from being formed that are less than about ten mils long. This drawback, coupled with the fact that wire bonds cannot be formed with acceptably tight tolerances at such dimensions, often results in unpredictable and/or unacceptable transmission characteristics for microwave devices.
In view of these drawbacks, some microwave device manufacturers have mounted microwave circuits using flip chip technology. Flip chip mounting has found wide application in the semiconductor packaging and assembly industry for digital and low frequency analog chips because it typically provides a cost and size reduction for the resulting semiconductor package. In contrast to the conventional wire bonding interconnect approach, the flip chip mounting technique involves flipping the chip and connecting the chip""s top surface to the substrate. A number of electrically conductive flip chip bumps, depending upon the complexity of the chip, are typically provided between the chip""s top surface and the substrate to provide an electrical connection between the chip and the substrate, and hence the other components connected to the top surface of the substrate.
In the microwave industry, however, because of difficulties in matching the orientation of the transmission mode fields for the circuits and the substrate, efforts to incorporate flip chip mounting have been primarily limited to devices employing co-planar waveguide (CPW) structures as the transmission media. That is, the circuit and substrate are both designed to support CPW. Many, if not most, commercially available MMICs (monolithic microwave integrated circuits), however, are designed for microstrip transmission modes, and are therefore ill suited for CPW transmission structures. Accordingly, using flip chip technology for microwave devices has ordinarily necessitated redesign or modification of existing microwave circuits to make them compatible with CPW. In addition, the CPW structure has the drawback that it typically requires the use of wire bonding to balance the ground strips of the CPW transmission line structure.
Accordingly, there exists a need for a lead frame package that provides the mechanical and electrical qualities necessary for high performance RF and microwave semiconductor devices, as well as for other types of devices. There further exists a need for such a package to be cost effective, both in terms of materials and assembly.
The present invention is directed to a method of packaging a device as well as an apparatus formed by the method. According to one embodiment, the apparatus includes a base connected to the device, and a cover. The cover includes a plastic body and at least one electrically conductive lead. The body of the cover is connected to the base such that the device is enclosed by the cover, and the electrically conductive lead includes an exposed portion that is electrically connected to the device.
According to another embodiment, the apparatus includes a device connected to an electrically conductive baseplate, and a cover. The cover includes a plastic body and at least one electrically conductive lead. The body of the cover is connected to the baseplate such that the device is enclosed by the cover, and the electrically conductive lead includes an exposed portion that is connected to the device via an electrically conductive bump.
The method, according to one embodiment, includes attaching the device to a base, and attaching the base to a cover, the cover including a plastic body and at least one electrically conductive lead, such that the body encloses the device and such that the exposed portion of the lead is electrically connected to the device.
In contrast to conventional semiconductor packaging techniques, the present invention provides a lead frame package that realizes the mechanical and electrical qualities necessary for high performance RF and microwave semiconductor. For example, the package of the present invention may provide an air gap that is ideally suited for devices requiring free spaces. In addition, embodiments of the present invention contemplate the use of materials that are ideally suited for high frequency applications, such as a low loss liquid crystal polymer injection molded plastic cover and a baseplate having a coefficient of thermal expansion (CTE) that matches the substrate material of the device. Furthermore, the leads of the package may be configured to provide controlled matching for high frequency applications. In addition to these performance benefits, the present invention provides cost advantages. The materials are relatively inexpensive, and the assembly process is ideally suited for high volume production and automation. These and other benefits of the present invention will be apparent from the detailed description hereinbelow.